Is this battery cycle life calculator correct?

[Usangira]

I have been roaming around in the internet researching about my life expectancy of my LiFePo4 battery bank.

I have come across this calculator here which is estimating my battery life to be 24years.

I am astonished by the numbers.... Can somebody validate it ?

https://app.calculoid.com/#/calculator/846

Thanks, Innocent

 

[Gazoo]

I don't know if it's accurate and I wonder what data it's based on. I use what most manufacturers use in their spec sheets. If a cell is fully cycled once per day at 1C rates, the cell will last apx. 5.5 years assuming the cell has a 2000 cycle life to 80% capacity. Of course the cells will last longer by keeping the SOC between the knees and I don't know of anyone who is running at full C rates. Temperature is also a big factor. I think many will find their cells aging before any other type of degradation as long as the cells are not abused.

 

[HRTKD]

I suspect it's a bit off. My LiFePO4 cells are in an RV trailer that gets actively lived in about six days a month, on average. I input what I thought were reasonable values and ended up with a battery life over 100 years. Yeah, I don't think so.

 

[Bob613]

They mention it is based on a particular Panasonic cell, in a paper published in 2015. Couldn't access the full paper so its not clear whether or not the analysis included correlation to actual testing.

However, one way of looking at lifetime is to take the manufacturer's cycle life info ( in my case, 3000 cycles from 100% down to 0%) and calculating the lifetime energy storage.
For a 280 A cell, the total lifetime storage is 280 amp hours * 3000 cycles =840,000 amp hours.
Using a 3.3 volt nominal , the energy stored is 840000*3.3= 2,772 kilowatt hours.
in my case, I have 24 cells, so the total is 2772*24=66,528 kilowatt hours.
In our case, we use (from the battery) an average of 6 KWhr per day, so I expect a life of 11,088 days, or about 30 years.
And at that point there should still be 80% of the capacity left.

Of course this doesn't take into account simple aging, which will, I expect come into play before the life cycles are used up.

 

[HOU_Castle]

I don't know if it's accurate and I wonder what data it's based on. I use what most manufacturers use in their spec sheets. If a cell is fully cycled once per day at 1C rates, the cell will last apx. 5.5 years assuming the cell has a 2000 cycle life to 80% capacity. Of course the cells will last longer by keeping the SOC between the knees and I don't know of anyone who is running at full C rates. Temperature is also a big factor. I think many will find their cells aging before any other type of degradation as long as the cells are not abused.

What is considered to be 1 cycle? Let say, at 9AM you have LFP battery (200Ah) fully charged connects to MPPT (to solar panel) and the battery connects to load. The load is 20A (constant) and Mppt charging 20A to battery. The sun sets at 7PM. From 9AM to 7PM is considered 1 cycle? or multiple cycles taking place due to charging and discharging activities?

 

[HRTKD]

If your load is 20 amp and your solar charge controller is providing 20 amps then your battery is doing . . . absolutely nothing. At night, without the solar charge controller, is a different matter. Your net is probably less than one cycle.

 

[grizzzman]

I'm willing to bet that ("side reactions") IE Calendar aging will kill the cells long before "light" cycling will.

 

[curiouscarbon]

I'm willing to bet that ("side reactions") IE Calendar aging will kill the cells long before "light" cycling will.

this is why i’m trying to make a crazy person peltier stacked plate cooler to regulate the temperature of my LFP batteries to 25C or less to experiment and see how long they will last me

 

[Gazoo]

What is considered to be 1 cycle?

A full cycle is discharging a fully charged cell from 3.65 volts to 2.50 volts, then charging it to 3.65 volts. That is one cycle.

 

[Bob613]

A full cycle is discharging a fully charged cell from 3.65 volts to 2.50 volts, then charging it to 3.65 volts. That is one cycle.

Consensus in the literature is that one cycle is the sum of how every many cycles it takes to equal 100% of the battery's capacity. For example, one day from full to 80%, another from 90% to 40%, and another from full to 70% adds up to 100% = one cycle.

 

[Sojourner1]

Consensus in the literature is that one cycle is the sum of how every many cycles it takes to equal 100% of the battery's capacity. For example, one day from full to 80%, another from 90% to 40%, and another from full to 70% adds up to 100% = one cycle.

This is how I understood it and how I figure my pack cycle count.
On my 500ah pack there are 1,750 partial DOD of 40% +/- at various SOCs for a rough combined full cycles of 700 so far.

 

[HOU_Castle]

Thank you for the explanation.

 

[curiouscarbon]

What is considered to be 1 cycle? Let say, at 9AM you have LFP battery (200Ah) fully charged connects to MPPT (to solar panel) and the battery connects to load. The load is 20A (constant) and Mppt charging 20A to battery. The sun sets at 7PM. From 9AM to 7PM is considered 1 cycle? or multiple cycles taking place due to charging and discharging activities?

How to count partial cycles is not entirely clear to me.

However.. I would count it piecewise.

Separately count Charging and Discharging Ampere Hours. And log temperature.

Graph cell voltage vs number of amp Hours charged and discharged at that voltage. Another graph for vs SOC based on coulomb counting.. And another with temperature.

Requires some specific logging/monitoring that I’m still in the process of programming for my own use.

Basically my logic…. the battery is a different chemical beast at different states of charge and voltage and temperature. By logging the voltage and SoC% and temperature while counting coulombs, I believe that I can collect enough information to empirically talk about “how many partial cycles” the pack has seen.

In my humble opinion, It’s combining the dataset described above, with a “counting algorithm” that the given person chooses, is how to most precisely discuss the aging of a partially cycled pack.

Of course, most don’t have this logging so it’s understandably challenging to get into the weeds so to speak, maybe.

This is a really complicated way of agreeing with you, haha.. anyways. Going repeatedly between 20-40% will probably be more wear than 40-60% repeatedly. But would be counted the same in some “counting algorithms”. By logging soc and counting coulombs I think it’s possible to be able to calculate a proper aging metric if that’s indeed the goal. I’ve ranted plenty. zzz ?

tl;dr i want to see every histogram of every pack of soc vs amp hours in and out to see if the pack is beat up

Cheers!

 

[curiouscarbon]

break up the charge voltage range into 100 evenly spaced bins. every time the coulomb counter registers 1 milliAmpHour it adds 1 to the bin that the battery voltage is currently at.

charging from 3.0 V would result in 1mAh by 1mAh being added to the lowest bins at first, then it hits 3.1 V and one by one milliamp hours are added to that counter.

after many months i reckon this histogram paints a very clear picture about how the battery pack is being worn down.

ensure that the counters can count very high if programming. 64-bit unsigned is probably good enough for anyone?

 

[curiouscarbon]

ah.. to tack on one more complication…

also log milliseconds spent charging at X amps at Y cell voltage.

if a 100Ah cell has logged many hundreds of thousands of milliseconds charging at 200A, then that cell might be toast. especially if done near the 2.5-3.0 V or 3.4-3.65 V range.

Adafruit SPI Non-Volatile FRAM Breakout - 64Kbit / 8KByte

FRAM, or Ferroelectric Ram, is the coolest new data storage method that all the fashion magazines are talking about. Oh wait, no that's quilted handbags. But FRAM is pretty damn cool ...

www.adafruit.com

6 usd 8 Kbyte

i am planning on using FRAM based storage like above link, to keep the histogram bin data safe between power off events.

Unlike Flash or EEPROM there's no pages to worry about. Each byte can be read/written 10,000,000,000,000 times so you don't have to worry too much about wear leveling.

Each byte can be read and written instantaneously (like SRAM) but will keep the memory for 95 years at room temperature.

 

[Gazoo]

Consensus in the literature is that one cycle is the sum of how every many cycles it takes to equal 100% of the battery's capacity. For example, one day from full to 80%, another from 90% to 40%, and another from full to 70% adds up to 100% = one cycle.

I posted what a full cycle is. I am not going to get into the weeds figuring partial cycles. Everyone will have to do that on their own because everyone's usage is different.

 

[DLTooley]

I’d worry more about all other risks over that time frame.

 

[grizzzman]

this is why i’m trying to make a crazy person peltier stacked plate cooler to regulate the temperature of my LFP batteries to 25C or less to experiment and see how long they will last me

Ya, I bought a Pelter set up also but I need to finish the insulation box before I set it up. How are you gonna deal with condensation?

 

[curiouscarbon]

Ya, I bought a Pelter set up also but I need to finish the insulation box before I set it up. How are you gonna deal with condensation?

Nice, you can do it!! Please post about it if/when you feel it’s appropriate ?

Planning on using temperature and relative humidity sensor on inside and outside of battery case. Calculate dew point in real-time. Never let peltier cold side go below dew point. Sensors from sparkfun and adafruit.

Using a brushless DC motor driver (h-bridge) to power multiple peltier connected in series. 9-10V power rail. H-bridge driver from pololu.

Since I expect the ambient air to become dry at least somewhat regularly, the battery compartment has two air ducts, Air Intake and Air Exhaust.

HEPA filter for vacuum cleaner for real cheap, cut one in half. Intake and exhaust both HEPA filtered ? using a small 12V blower 5W to push air out of the box when internal relative humidity is above threshold and ambient relative humidity is low. Humidity still creeps in slowly on days timescale, but is easily purged conditionally.

 

[Gazoo]

Planning on using temperature and relative humidity sensor on inside and outside of battery case. Calculate dew point in real-time. Never let peltier cold side go below dew point. Sensors from sparkfun and adafruit.

I had totally forgotten about the dew point. Years ago I bought some tiny peltiers to experiment with to cool a hobby laser. I found a dew point chart so I used that to keep condensation in check. I think you have a "cool" idea to do this automatically. I look forward to your findings.